1. Field of the Invention
The present invention relates to launch vehicles for placing payloads into earth orbit or higher energy orbits which permit the payload to escape from earth. More particularly, the present invention relates to multistage launch vehicles.
2. Description of Prior Art and Related Information
The various approaches to launch vehicle design may be generally classified into single stage and multistage launch vehicle systems. Single stage launch vehicles employ a single thruster stage which includes all the propellant required to deliver a specified velocity to the payload. Since considerable mass is contained in the propellant tanks, engines and thrust structure, which mass becomes unnecessary once propellant therein is expended, a single stage launch vehicle is inherently of less than optimum efficiency. Multistage launch vehicles have accordingly been developed and gained predominance for earth orbit launch applications. Due to their simplicity, however, single stage launch vehicles will in general be cheap and more reliable than multistage launch vehicles.
Accordingly, a single stage vehicle with multiple engines (Atlas) has been utilized in a mode where only engines and part of their thrust structure have been staged at an appropriate time in flight (commonly called a stage and a half, 1.5 stage). The 1.5 stage has two important features: (1) it reduces the weight at a time, in flight when the jettisoned weight is no longer necessary to the efficient performance of the stage, and (2) it reduces the thrust at a time when the propellant weight has been reduced to the point that, with all engines continuing to thrust, the acceleration loads (thrust to weight ratio) delivered to the stage and its payload would be greater than desired from a design standpoint.
Multistage launch vehicles may be characterized in two general categories, serial staged and parallel staged launch vehicles. Serial staged launch vehicles, also referred to as tandem staged launch vehicles, employ two or more vertically aligned stages mounted on top of each other and coupled together in a manner allowing separation during staging. Each stage includes two propellant tanks, a fuel and an oxidizer tank, rocket engine and thrust structure. Upon launch only the lower or first stage is ignited. (The numbering of stages as used herein is in accordance with the convention that the first stage to be separated is referred to as stage 1, and subsequent stages numbered consecutively thereafter in order of separation.) Once the propellant in the first stage is exhausted, the first stage is separated from the second and remaining stages and the second stage is ignited. For an N stage launch vehicle, this staging is repeated N-1 times. For example, for a three stage space launch vehicle, after the expenditure of propellant in the second stage, it is staged and the third stage is ignited and lifts the payload into orbit.
Due to the reduction of weight in the separation of the expended fuel tanks, engine(s) and structure, the series staged launch vehicle is currently employed for many military and civilian applications. Despite the advantages of a series staged launch vehicle, however, series staging inherently has some disadvantages associated therewith.
First of all, only the first stage is ignited at lift off of the launch vehicle and therefore the second and subsequent stages must be ignited upon separation of the prior stages. Since the igniting and early operation of a rocket engine is the most likely time for a failure to occur, the ignition of stages in series introduces increased susceptibility to propulsion failures. Also, a propulsion failure in any stage after the first stage will generally result in mission failure. This is in contrast to a stage and a half parallel staged launch vehicle (discussed below) where the rocket engines are ignited on the ground and the launch can be aborted in the event of a propulsion problem, thus avoiding the loss of the launch vehicle. Secondly, the amount of thrust at lift-off is limited in a series staged launch vehicle to the engine thrust in the first stage. Such limited thrust puts limitations on the amount of payload which may be carried into orbit.
An alternate approach to series staging, parallel staging, employs two or more stages which are ignited at lift off. One or more of the parallel stages are subsequently separated from the payload stage for reduction in overall weight of the launch vehicle during the latter stages of the launch. Such a parallel stage launch vehicle is illustrated in FIG. 1 for the case of a two stage launch vehicle. As shown in FIG. 1, the two stages, stage 1 and stage 2, are placed in a side-by-side or "parallel" manner with the payload mounted on the second stage. The two stages are coupled together by a releasable mechanism illustrated by struts 2 and 4. At launch, the rocket engines of both stage 1 and stage 2 are ignited simultaneously. As shown in FIG. 1, the thrust and propellant tanks of the first stage are generally sized so that it will be depleted of propellant at a time when substantial propellants still remain in stage 2. At this time, the struts 2 and 4 are separated, releasing the first stage and allowing the second stage and payload to continue on into orbit without the added weight of the expended propellant tanks, engines and thrust structure of the first stage. The rocket engines in the first and second stages may also be of different type, so that one stage burns faster than the other. For example, the space shuttle employs two fast burning solid rocket boosters ignited in parallel with the main engines at lift off, which solid rocket boosters are staged after the propellent therein is expended.
The parallel stage launch vehicle approach has some advantages relative to a series staged launch vehicles, nonetheless trade offs result and none of these systems is optimal in all respects. With respect to the parallel stage launch vehicle, the parallel ignition of the rocket engines at lift off increases the thrust at lift off over a series staged with the same total thrust. Since a general condition on any space vehicle launch is that the ratio of its thrust to gross weight, at lift off, should be of the order of 1.2 to 1.6, a parallel stage system can have a larger gross weight, and hence carry more propellant, than a series staged vehicle with the same total thrust. On the other hand, the parallel staged system is less efficient since the weight of the propellant tanks of the second stage is increased over a series stage system since the second stage burns continuously from lift off and hence must carry more propellant than if burned in series. Also, a parallel stage system avoids the problem of igniting second stage engines in flight and the added risk factor associated therewith. Nonetheless, potential problems remain due to the multiple stages in that catastrophic failure could occur in any one of the stages. Due to the increased awareness of the risks of space flight after the Challenger disaster, the importance of which has pervaded the entire space program in recent years, any possible increase in reliability, and hence safety, of a multistage launch vehicle is of the utmost importance.
Accordingly, a need presently exists for a launch vehicle system and method which can further increase the safety and reliability of the launch vehicle. Additionally, a need presently exists for a launch vehicle system and method which can optimize both thrust and performance in such an increased reliability launch vehicle.